Uploading from the admin panel

@Marjolaine was describing an issue where the client uploads a logo using the sib-form on the front-end. They can edit the logo to an existing web url (e.g. from Google images), but they can't select a local file to upload, like so:

How to do this now This is possible to work-around

1. Select add new DjangoLDP_Uploader Document in the admin panel

2. Select the file to upload and then select 'save and continue editing'

3. Copy the URL id:

4. Return to the model, and paste the URL id into the logo field, and save

A cleaner solution

1. Make a new Field UploadedDocumentURL, which extends Django's URLField, without changes
2. Override the form-field of UploadedDocumentURL to provide a custom widget which displays a file upload alongside a URL text input (the current value displayed here)

Estimation: 2-4 hours

@sylvain @balessan what do you think? This is perhaps a lot of coding to improve the admin panel UX